linux-kernel - Re: [PATCH v1 0/9] Fix MMC properties on Sitara devices

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Date: Mon, 12 Feb 2024 11:31:36 -0600
From: Judith Mendez <jm@...com>
To: Francesco Dolcini <francesco@...cini.it>
CC: Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski
	<krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>, Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Tero
 Kristo <kristo@...nel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 0/9] Fix MMC properties on Sitara devices

Hi Francesco,

On 2/11/24 9:44 AM, Francesco Dolcini wrote:
> Hello Judith and TI folks,
> 
> On Wed, Feb 07, 2024 at 04:55:17PM -0600, Judith Mendez wrote:
>> This patch series aims to add or fix MMC properties:
>> OTAPDLY/ITAPDLY.
>>
>> The DLL properties ti,trm-icp and ti,driver-strength-ohm
>> should also be updated since only AM64x and AM62p devices
>> have a DLL to enable, so remove these properties when not
>> applicable.
> 
> Do you have any reference regarding this change? TI reference manual or
> anything like that?

I believe there is no specific documentation that states outright that
these device tree properties are not applicable for devices like AM62x.

There are a few hints in the device datasheet and TRM, if the MMC PHY
has a DLL to enable, DLL properties should show in both docs.

Also, you can read the MMC bindings doc description:
https://github.com/torvalds/linux/blob/master/Documentation/devicetree
/bindings/mmc/sdhci-am654.yaml#L179

> 
> No change needed in sdhci_am654.c? It seems that `drv_strength` is written
> to some register unconditionally, is it ok to do so?

Actually, we only write this value to the register if there is a DLL
to enable. If we have this property in device tree for devices like
AM62x and AM62ax, this property is ignored in the driver. So I am
removing this property from device tree as part of the effort to
clean up MMC nodes in device tree.

> 
> Do this change implies that there is no way to configure the drive
> strength on such SoCs and MMC/SD trace impedance must be the nominal
> 50ohm?

We do not support changing drive strength for these devices. My 
understanding is that the drive strength should default to about
40 ohms, which should be good enough for any eMMC operating mode.

~ Judith

> 
> Thanks,
> Francesco
>